The reflective cholesteric texture displays have two stable optical states at zero field. One is the planar texture which reflects light at a preselected wavelength determined by the pitch of the cholesteric liquid crystal. The other is the focal conic texture which is weak, scattering or nearly optically transparent.
There are two ways to achieve bistabilty in the cholesteric displays: one is polymer network and the other is the surface condition. The polymer stabilized cholesteric texture (PSCT) displays are made from panels with homogeneous surface treatment, filled with the cholesteric liquid crystal mixture with small amounts of monomer and photoinitiator, and cured by ultraviolet irradiation in the presence of an electric field which aligns the molecules perpendicular to the substrate. The surface stabilized or polymer free cholesteric displays are made without homogenous surface treatment and polymer gel. PSCT liquid crystal displays are fully described in U.S. Pat. Nos. 5,251,048 and patent application Ser. Nos. 07/694,840 and 07/969,093.
The polymer free displays require much simplified manufacturing processes, but have not yet been used in production because of the difficulties in controlling the inactive areas. The inactive areas would be in bright reflecting state due to the flow orientation, which is very irritating for medium to high resolution displays. It degrades the black state and hence reduces the contrast. We can make the inactive areas black by thermal annealing in the polymer free displays. However, the texture is very sensitive to mechanical shocks. The inactive areas change into bright reflective state after being squeezed. As a result, the background color of the display is non-uniform. The reflectivity of the inactive area of the PSCT is relatively insensitive to mechanical shocks since the molecules are in the planar state. However, the bright color in the active area makes the dark portion of a displayed image less dark and hence reduces the image contrast for medium to high resolution displays.
The problems described above can be overcome by the application of black mask inside the cell in principle. However, black mask is thick (greater than 1 .mu.m) and difficult to make for high resolution displays. It also adds manufacturing cost and introduces potential contaminants into the cells. In addition, the non-smooth surface caused by black mask may generate extra chromatographic problems during filling.